SUMMARY OF EXISTING INFORMATION ON THE INCOHERENT-SCATTERING OF PHOTONS, PARTICULARLY ON THE VALIDITY OF THE USE OF THE INCOHERENT-SCATTERING FUNCTION

X-ray scattering by individual atomic electrons is known as Compton sc attering if the simplifying approximation is invoked that the target e lectrons are initially free and at rest, in which case the differentia l and integral scattering cross sections can be computed using the com pact analytical expressions given by Klein and Nishina in 1929. In rea l atoms the electrons are neither free nor at rest, resulting in depar tures from the Klein-Nishina expressions. These departures have been e stimated theoretically in varying degrees of sophistication, with corr espondingly varying degrees of ease in applying these corrections to t he free electron model. At present the most widely used treatment of t hese departures is via the incoherent scattering function S(x,Z) in wh ich x is a momentum transfer variable dependent on the incident photon energy and the scattered-photon deflection angle, and Z is the atomic number of the atom associated with the target electron. The incoheren t scattering function S(x,Z) also enters into calculations of the cros s section for electron-positron pair production in the fields of the a tomic electrons, known also as tripler production. This report include s a historical survey of experimental and theoretical work on the Comp ton effect and on the incoherent scattering function. The validity of the use of the incoherent scattering function is examined and discusse d. Published by Elsevier Science Ltd.